Step 1: Understanding the Question:
The question asks for the primary counting criterion used in the "de facto" method of taking a national census.
Step 2: Detailed Explanation:
De Facto Method: This is also known as the "one-night enumeration" or "snapshot" method. Every person is counted at the exact location where they are physically present at the time of the census count (usually a single reference night). If someone is in a hotel or on a train, they are counted at that spot.
De Jure Method: In this system, individuals are counted at their usual or permanent place of residence, regardless of where they happen to be on the census night.
Comparison:
De Facto (Option B): Simpler and faster but can provide distorted data for local planning if many people are traveling.
De Jure (Option D): More difficult to execute but better reflects the actual residency and infrastructure needs of an area. India uses an "extended de jure" method.
Outcome: Since "de facto" means "in fact" or "actually present", the census count is based strictly on physical location during enumeration.
Step 3: Final Answer:
The de facto method counts every individual based on where they are physically found during the census night.